Casing for a wellbore that has just been drilled is assembled at the surface as joints are added and the string is lowered into the wellbore. As the joints are added at the surface on the rig floor, it is often desirable to fill the casing with fluid or drilling mud. Filling the casing before it is run into the wellbore prevents pressure imbalances on the casing as it is being advanced into the wellbore. Additionally, once the casing is filled, it may be desirable to circulate through the casing as it is being run into the wellbore. Thus, it is often necessary to use an apparatus for filling and circulating fluids within the casing. When such an apparatus is raised from the casing, fluids may leak onto the well deck, which wastes valuable fluids, may be hazardous to personnel, and could cause environmental issues. Furthermore, such an apparatus may build up excessive back pressure causing potentially dangerous situations. What is needed, therefore, is an apparatus and method which safely allows for the adequate filling and circulating of the casing.